1) there are several ways. You can start by downgrading your modem to its original since you are still on older firmware and see if that works
2) not very risky, just make sure you know about Knox and all that
3) Nope, unlocked for good

Hello everyone, I m from India and i recently got a samsung sgh m919 from my ajnt and got it unlocked. The person who did it, rooted my device in order to unlock. Now i want to know if 1) Can unrooting remove sim unlock? 2) can i install any custom rom without loosing sim unlock? Thank you

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